This invention relates to a novel finish for metals or ceramics.
It is well known that fluorocarbon polymers have excellent physical properties such as low abrasiveness, non-tackiness and high resistances to chemicals and heat and are used in a wide variety of fields such as domestic uses (such as in irons, frypans, etc.), the food industry, the electric industry, the machine industry and like industries. However, due to their inherent non-tackiness, fluorocarbon polymers are very poor in adhesiveness to all kinds of materials. Thus, this property makes it much more difficult to coat substrates with such polymers as compared with other known polymers. In order to overcome the above difficulty, there have been proposed several methods including a method in which a primer coat is applied onto a substrate prior to the coating of a fluorocarbon polymer topcoat, ensuring strong adhesion of the topcoat to the substrate through the primer coat, a method in which a substrate surface is chemically or physically roughened to facilitate adhesion of the fluorocarbon polymer to the roughened substrate by a so-called anchoring effect, and a method in which a fluorocarbon polymer per se is improved so as to enhance its adhesiveness to substrates.
Since known primer compositions generally contain strong acids or strong alkalis, they encounter several problems such as pollution of working environments upon preparation or during use of such primer composition and wastage of such compositions generally occurred in practical application.
In recent years, polyarylene sulfide resins (hereinafter referred to simply as PAS), typical of which is polyphenylene sulfide, ##STR1## (hereinafter referred to simply as PPS), have attracted particular attention due to their characteristic properties of high resistance to heat and high affinity for fluorocarbon polymers. For example, in Japanese patent publication No. 51-31813/1976, there is proposed a method in which polyphenylene sulfide resin is sprayed over a blasted metal surface and baked to form a primer coat on which a fluorocarbon polymer topcoat is formed. Further, there are also known, from Japanese patent publication No. 12053/1976 and German Offenlegungsschrift No. 2152770, primer compositions containing polyphenylene sulfide and a fluorocarbon polymer as principal components particularly suitable for applying a fluorocarbon resin overlay coating.
In the former method in which a primer composition consisting essentially of particulate polyphenylene sulfide resin is used, it is essential that baking be effected at a relatively low temperature of 230.degree.-320.degree. C., in order to obtain satisfactory adhesion between a fluorocarbon polymer topcoat and the primer coat. However, such low temperature baking will not give a satisfactory adhesion between the substrate and the primer coat. When the PPS is baked at a high temperature to permit the primer to strongly adhere to the substrate, the resulting primer coat will become very poor in adhesion to the fluorocarbon polymer topcoat. Thus, the primer coat consisting essentially of PPS is not practical. Furthermore, though the PPS primer coat has a satisfactory hardness at a normal temperature, its hardness is low at a high temperature of, for example, about 200.degree. C. Under such high temperature conditions the primer coat tends to be damaged with the attendant problem of lower resistance to scratch.
In the latter case where PPS admixed with a fluorocarbon polymer is used as a primer composition, adhesion between the primer and the fluorocarbon polymer topcoat is improved. However, adhesion between the primer coat and its metal substrate will be naturally lowered. Accordingly, in order to ensure a practical, usable adhesion between the primer coat and the metal substrate, it is essential to have the content of the fluorocarbon polymer below 20% by weight of the primer composition. However, such compositions do not yet provide satisfactory adhesion between a metal substrate and the primer coat or between the primer coat and the topcoat. Further, the addition of a fluorocarbon polymer to a primer composition is disadvantageous in that the final coated article is lowered in resistance to corrosion because of insufficient bonding between the substrate and primer coat.